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        <h3 id="单例模式"><a href="#单例模式" class="headerlink" title="单例模式"></a>单例模式</h3><p>这个模式其实并不需要过多的阐释，重点是如何构建好一个单例模式，最基础的代码如下</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">package</span> org.kys.Gaf.singleton;</span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Singleton</span> </span>&#123;</span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">final</span> Singleton = <span class="keyword">new</span> Singleton();</span><br><span class="line">    <span class="function"><span class="keyword">private</span> <span class="title">Singleton</span><span class="params">()</span> </span>&#123;&#125;</span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="title">getSignleton</span><span class="params">()</span></span>&#123;</span><br><span class="line">        <span class="keyword">return</span> singleton;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>以上的方法属于饿汉模式：一开始就生成的实例</p>
<p>其实单例模式还有一种懒加载模式-饿汉模式，不过这种模式有许多的坑</p>
<h3 id="懒加载单例模式最基础样式"><a href="#懒加载单例模式最基础样式" class="headerlink" title="懒加载单例模式最基础样式"></a>懒加载单例模式最基础样式</h3><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">package</span> org.kys.Gaf.singleton;</span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Singleton</span> </span>&#123;</span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">static</span> Singleton singleton = <span class="keyword">null</span>;</span><br><span class="line">    <span class="function"><span class="keyword">private</span> <span class="title">Singleton</span><span class="params">()</span></span>&#123;&#125;</span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> Singleton <span class="title">getSingleton</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        <span class="keyword">if</span>(singleton == <span class="keyword">null</span>) singleton = <span class="keyword">new</span> Singleton();</span><br><span class="line">        <span class="keyword">return</span> singleton;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<blockquote>
<p>这种写法有线程安全问题，比如1线程运行到if语句中2线程也运行到if中，之后将会创建两个对象造成对象不同。</p>
</blockquote>
<h3 id="添加线程安全的单例模式"><a href="#添加线程安全的单例模式" class="headerlink" title="添加线程安全的单例模式"></a>添加线程安全的单例模式</h3><ol>
<li>基本思路 synchronized解决</li>
</ol>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">package</span> org.kys.Gaf.singleton;</span><br><span class="line"></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Singleton</span> </span>&#123;</span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">static</span> Singleton singleton = <span class="keyword">null</span>;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">private</span> <span class="title">Singleton</span><span class="params">()</span> </span>&#123;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">synchronized</span> Singleton <span class="title">getSingleton</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        <span class="keyword">if</span> (singleton == <span class="keyword">null</span>) singleton = <span class="keyword">new</span> Singleton();</span><br><span class="line">        <span class="keyword">return</span> singleton;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<blockquote>
<p>缺点性能低下 synchronized 99% 不需要线程同步</p>
</blockquote>
<ol start="2">
<li>synchronized代码块优化</li>
</ol>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">package</span> org.kys.Gaf.singleton;</span><br><span class="line"></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Singleton</span> </span>&#123;</span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">static</span> Singleton singleton = <span class="keyword">null</span>;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">private</span> <span class="title">Singleton</span><span class="params">()</span> </span>&#123;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> Singleton <span class="title">getSingleton</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        <span class="keyword">synchronized</span>(Singleton.class) &#123;</span><br><span class="line">            <span class="keyword">if</span> (singleton == <span class="keyword">null</span>) singleton = <span class="keyword">new</span> Singleton();</span><br><span class="line">        &#125;</span><br><span class="line">        <span class="keyword">return</span> singleton;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<blockquote>
<p>没有使用全局锁但是还是有问题，代码块同样需要每次运行</p>
</blockquote>
<ol start="3">
<li>双重锁家线程可见</li>
</ol>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">package</span> org.kys.Gaf.singleton;</span><br><span class="line"></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Singleton</span> </span>&#123;</span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">volatile</span> <span class="keyword">static</span> Singleton singleton = <span class="keyword">null</span>;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">private</span> <span class="title">Singleton</span><span class="params">()</span> </span>&#123;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> Singleton <span class="title">getSingleton</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        <span class="keyword">if</span> (singleton == <span class="keyword">null</span>) &#123;</span><br><span class="line">            <span class="keyword">synchronized</span> (Singleton.class) &#123;</span><br><span class="line">                <span class="keyword">if</span> (singleton == <span class="keyword">null</span>) singleton = <span class="keyword">new</span> Singleton();</span><br><span class="line">            &#125;</span><br><span class="line">        &#125;</span><br><span class="line">        <span class="keyword">return</span> singleton;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>这里使用两个if 来解决99%情况下不需要对if进行线程隔离的情况，使用volatile是为了防止指令重拍续导致的问题，分析如下：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">static</span> Singleton <span class="title">getSingleton</span><span class="params">()</span> </span>&#123;</span><br><span class="line">    <span class="keyword">if</span> (instance == <span class="keyword">null</span>) &#123;                         <span class="comment">//Single Checked</span></span><br><span class="line">        <span class="keyword">synchronized</span> (Singleton.class) &#123;</span><br><span class="line">            <span class="keyword">if</span> (instance == <span class="keyword">null</span>) &#123;                 <span class="comment">//Double Checked</span></span><br><span class="line">                instance = <span class="keyword">new</span> Singleton();</span><br><span class="line">            &#125;</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="keyword">return</span> instance ;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>这段代码看起来很完美，很可惜，它是有问题。主要在于instance = new Singleton()这句，这并非是一个原子操作，事实上在 JVM 中这句话大概做了下面 3 件事情。</p>
<ol>
<li>给 instance 分配内存</li>
<li>调用 Singleton 的构造函数来初始化成员变量</li>
<li>将instance对象指向分配的内存空间（执行完这步 instance 就为非 null 了）</li>
</ol>
<p>但是在 JVM 的即时编译器中存在指令重排序的优化。也就是说上面的第二步和第三步的顺序是不能保证的，最终的执行顺序可能是 1-2-3 也可能是 1-3-2。如果是后者，则在 3 执行完毕、2 未执行之前，被线程二抢占了，这时 instance 已经是非 null 了（但却没有初始化），所以线程二会直接返回 instance，然后使用，然后顺理成章地报错。</p>
<h3 id="使用内部静态类"><a href="#使用内部静态类" class="headerlink" title="使用内部静态类"></a>使用内部静态类</h3><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Singleton</span> </span>&#123;  </span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">static</span> <span class="class"><span class="keyword">class</span> <span class="title">SingletonHolder</span> </span>&#123;  </span><br><span class="line">        <span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">final</span> Singleton INSTANCE = <span class="keyword">new</span> Singleton();  </span><br><span class="line">    &#125;  </span><br><span class="line">    <span class="function"><span class="keyword">private</span> <span class="title">Singleton</span> <span class="params">()</span></span>&#123;&#125;  </span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">final</span> Singleton <span class="title">getInstance</span><span class="params">()</span> </span>&#123;  </span><br><span class="line">        <span class="keyword">return</span> SingletonHolder.INSTANCE; </span><br><span class="line">    &#125;  </span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>这种写法仍然使用JVM本身机制保证了线程安全问题；由于 SingletonHolder 是私有的，除了 getInstance() 之外没有办法访问它，因此它是懒汉式的；同时读取实例的时候不会进行同步，没有性能缺陷；也不依赖 JDK 版本。</p>
<h3 id="枚举-Enum"><a href="#枚举-Enum" class="headerlink" title="枚举 Enum"></a>枚举 Enum</h3><p>用枚举写单例实在太简单了！这也是它最大的优点。下面这段代码就是声明枚举实例的通常做法。<br><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="keyword">enum</span> EasySingleton&#123;</span><br><span class="line">    INSTANCE;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure></p>
<p>我们可以通过EasySingleton.INSTANCE来访问实例，这比调用getInstance()方法简单多了。创建枚举默认就是线程安全的，所以不需要担心double checked locking，而且还能防止反序列化导致重新创建新的对象。但是还是很少看到有人这样写，可能是因为不太熟悉吧。</p>
<h3 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h3><p>一般来说，单例模式有五种写法：懒汉、饿汉、双重检验锁、静态内部类、枚举。上述所说都是线程安全的实现，文章开头给出的第一种方法不算正确的写法。</p>

      
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              <div class="post-toc-content"><ol class="nav"><li class="nav-item nav-level-3"><a class="nav-link" href="#单例模式"><span class="nav-number">1.</span> <span class="nav-text">单例模式</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#懒加载单例模式最基础样式"><span class="nav-number">2.</span> <span class="nav-text">懒加载单例模式最基础样式</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#添加线程安全的单例模式"><span class="nav-number">3.</span> <span class="nav-text">添加线程安全的单例模式</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#使用内部静态类"><span class="nav-number">4.</span> <span class="nav-text">使用内部静态类</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#枚举-Enum"><span class="nav-number">5.</span> <span class="nav-text">枚举 Enum</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#总结"><span class="nav-number">6.</span> <span class="nav-text">总结</span></a></li></ol></div>
            

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